Transflective liquid crystal displays (LCDs) utilize some ambient light to illuminate active matrix pixels when operating in bright light environments (i.e. high ambient light environments). The direction of the ambient light's specular component requires diffusion or randomization to prevent mirror images off of the surroundings and to enlarge the useful illumination cone. Certain prior art transflective displays have utilized a diffusing layer disposed outside of the display's polarizers (i.e. not between the polarizers and not between the substrates). Strong depolarizing effects and large thicknesses of conventional diffusers have forced such placement outside of the substrates and outside of the polarizers.
For example, see prior art FIG. 1 which is taken from commonly owned U.S. Pat. No. 5,629,784. As illustrated, the FIG. 1 display includes from the rear forward linear polarizer 1, transparent substrate 3, discrete pixel electrodes 7, rear orientation or buffing film 9, liquid crystal (LC) layer 11, front orientation or buffing film 13, common electrode 15, front substrate 17, front linear polarizer 19, optical film 21 including facets 23, holographic diffuser 25, and finally glass sheet 27. It is noted that diffuser 25 is located on the front side of the LC layer and outside of the polarizers 1, 19, and also outside (i.e. exterior) of the display's substrates 3, 17. Liquid crystal layer 11, electrodes 7, 15, and orientation films 9, 13 are located between the substrates, and between the polarizers.
Unfortunately, prior art diffusing layers, including that of the '784 patent shown in FIG. 1 herein, suffer from at least the following problems: (i) substantial depolarization of light, (ii) image parallax, and (iii) production problems which prevent practical use between a display's substrates.
With regard to depolarization shortcomings of prior art diffusing layers, such diffusers utilize scattering of input light rays to diffuse or randomize the direction of the output light rays. Conventional scattering mechanisms can be surface roughness as disclosed in the '784 patent. Certain types of scattering effects substantially depolarize light traveling through the display. This is disadvantageous, in that proper polarization is required for efficient LCD operation, in view of the typically utilized front and rear linear polarizers. Thus, each of volume diffusers, holographic diffusers, and reflective diffusers provide sizes of scatter which are on the order of a wavelength of light or smaller, and can create an undesirable depolarizing effect.
Prior art LCD diffusing systems also create parallax problems. A light diffuser or diffusing layer on the outside of a display's substrates, or on the outside of a display's polarizers, creates a detrimental effect called image parallax or pixel crosstalk. Prior art FIG. 2 illustrates the cause of such image parallax or pixel crosstalk in an LCD. Shown in FIG. 2 are diffuser 37, 25, incident light ray 31, pixel aperture 33 having a size or width "w", and pixel acceptance cone angle 35. The direction of incident light rays 31 striking diffuser screen 37 is randomized which increases the useful illumination cone 35 and prevents mirror images of the surrounding environment. The separation "d" of diffusing layer 37, 25 from pixel aperture(s) 33 allows some light 39 to cross over and exit through adjacent pixels thereby blurring the resulting image. Image parallax worsens as the separation "d" between diffusing layer 37 and pixel aperture(s) 33 increases because more light 39 can make its way into adjacent or distant pixels. Pixel apertures 33 are typically defined proximate the liquid crystal (LC) layer between or at the pixel electrodes. Thus, it would be desirable to have the diffuser as close to the LC layer as possible.
In furtherance of the above, prior art FIG. 3 illustrates a conventional transflective LCD configuration. The distance "d" between diffuser 37, 25 and pixel aperture(s) 33 is approximately 1,100 .mu.m. A large portion of this 1,100 .mu.m distance "d" is defined by the thickness of glass substrate 17. This large separation "d" creates substantial image parallax or pixel crosstalk due to the large distance that rays 39 can travel laterally from their correct or originating pixel. The strong depolarizing effect and large thickness of conventional diffusers 37, 25 forces their placement as illustrated in FIG. 3 on the outside of the display's substrates, and on the outside of the display's polarizers.
Prior art mass production methods of conventional diffusers are also not compatible with placing known diffusers between glass substrates of a display. For example, mass production of holographic diffusers 25 involves a film embossing process. The manufacturing of conventional diffusers requires the roughening of surfaces by physical (e.g. sanding) or chemical (e.g. etching) processes. These methods would not be efficiently utilized in providing a diffuser between either opposing polarizers or substrates of an LCD.
It is apparent from the above that there exists a long felt need in the art for a liquid crystal display (e.g. normally white, normally black, active, TN, STN, etc.) with a diffuser layer(s) provided so as to (i) reduce image parallax or pixel crosstalk, (ii) minimize depolarizing effects, and/or (iii) be manufacturable in mass production by way of a method so as to be efficiently placeable in between substrates or polarizers of a display without undue cost or sacrificing of yields. It is a purpose of this invention to satisfy the above-described needs in the art.
This invention will now be described with respect to certain embodiments thereof, accompanied by certain illustrations.